It is known from the state of the art that organo-modified siloxanes, such as polyether siloxanes or polysiloxanes, having substituents with anionic, cationic or amphoteric groups, an appropriately selected structure and a balanced ratio of hydrophilic to hydrophobic groups, can lower the surface tension of aqueous solutions to a pronounced degree.
Surfactants with at least three silicon atoms are described in the German patent 41 41 046. They correspond to the general formula ##STR2## wherein R.sup.1 are methyl or phenyl groups, with the proviso that at least 90% of the R.sup.1 groups are methyl groups,
R.sup.2 is identical with R.sup.1 or --(CH.sub.2).sub.6 --SO.sub.3.sup.-.M.sup.+, wherein M.sup.+ is an alkali, 1/2 an alkali earth or optionally an alkyl-substituted ammonium ion, PA1 R.sup.3 is identical with R.sup.1 or R.sup.2, with the proviso that at least one R.sup.2 or R.sup.3 group in an average molecule is a --(CH.sub.2).sub.6 --OSO.sub.3.sup.-.M.sup.+ group, PA1 a has a numerical value of 0 to 5, and PA1 b has a numerical value of 0 to 5. PA1 R' is an alkyl group with 1 to 6 carbon atoms, PA1 R" is a divalent aliphatic hydrocarbon group with 2 to 6 carbon atoms, which connects Q and the adjacent silicon atom by means of a bridge of at least 2 carbon atoms, PA1 Q is the --O(C.sub.2 H.sub.4 O).sub.c X group, wherein c has a value of 3 to 12 and X is a hydrogen group, PA1 R"' is ##STR5## in which R"' is an alkyl group with 1 to 5 carbon atoms, and a=1 or 2 and b=2 or 3. PA1 R.sup.4 is a divalent hydrocarbon group with 3 to 14 carbon atoms, PA1 R.sup.5 is a group having the formula --O(CH.sub.2).sub.b -- or a polyether group having the formula --(OC.sub.n H.sub.2n).sub.c --, wherein PA1 a is 0 or 1, PA1 wherein R.sup.6 is a divalent, aliphatic hydrocarbon group with 1 to 12 carbon atoms, or PA1 b) compounds of the general formula EQU CH.sub.2 .dbd.CH--R.sup.6 --OH
In neutral, aqueous media, the selected trisiloxanehexyl sulfates having three silicon atoms greatly decrease the surface tension of neutral media to values of about 21 mN/m. However, in acidic or alkaline solutions, they are not stable and, due to hydrolysis of the Si--O--Si bonds and renewed condensation of the hydrolysis products to higher molecular weight oligomers, very rapidly lose their effectiveness and partly become insoluble in aqueous media.
Surfactants with a low content of silicon atoms are furthermore described in the European publication 0 367 381 (A2) and in the British patent 1,520,421.
The European publication 0 367 381 (A2) relates to organosilicon compounds of the general formula ##STR3## wherein R independently of one another represent an alkyl, aryl, halogenated alkyl or halogenated aryl group with up to 18 carbon atoms, each R' represents an alkylene group, which separates adjacent silicon atoms from one another by up to 6 carbon atoms and R" independently of one another represent R or, when a is equal to zero, the R.sub.3 SiR' group. Z is a hydrophilic substituent, which contains sulfur, nitrogen or phosphorus, a carboxy-functional group or its salt. a has a value of 0, 1 or 2.
It follows from this that the organosilicon group, by definition, contains at least two silicon atoms. The synthesis of these carbosilanes is relatively expensive and is accomplished, for example, by a method similar to a Grignard reaction. After that, carbosilane surfactants, with a quaternary, sulfonate or betaine structure, are synthesized by means of a hydrosilylation of, for example, allyl glycidyl ether or allylamine and well-known subsequent reactions. The substances, so obtained, lower the surface tension of a 1% solution in distilled water to 23 to 25 mN/m.
In the British patent 1,520,421, carbosilane surfactants and their synthesis are described. They have the general formula ##STR4## wherein R is a methyl, ethyl, propyl or trifluoropropyl group, with the proviso that at least 50% of the R groups are methyl groups,
According to definition, at least two silicon atoms must be present here also. In application tests, these compounds exhibit remarkable foaming properties.
It was known to those skilled in the art that the surfactant properties of the compounds within groups of known carbosilanes with comparable structure deteriorate as the number of silicon atoms is decreased, in particular, as the number of silicon atoms is decreased from 4 to 3 or 2. This observation is embodied in the theory of Neumann (A. W. Neumann, D. Renzow, Zeitschrift f. Phys. Chem., new issue 68, 11 (1969)), which states that the permethylated surface of the siloxane backbone is responsible for the lowering of the surface tensions of aqueous solutions to values below 30 to 40 mN/m.
Furthermore, reference is made to the Japanese publications of H. Maki et al. in YUKAGAGU 19, No. 4, page 51 ff. and YUKAGAGU 19, No. 11, page 23 ff., both from 1970, wherein defined compounds of the formula EQU (CH.sub.3 (CH.sub.2).sub.3 Si(CH.sub.2).sub.3 (C.sub.2 H.sub.4 O).sub.m H
are described, in which n=4.2 or 7.0 and m=10 or 17. However, these compounds lower the surface tension of a 1% by weight solution only to values not less than 26.5 mN/m.
In these Japanese publications, quaternary nitrogen compounds of the formula EQU Bu.sub.3 M(CH.sub.2).sub.3 N.sup.+ (CH.sub.3).sub.3 Cl.sup.- (Bu=Butyl, M=Sn, Si)
are also described. Admittedly, these compounds have bacteriostatic activity; however, they are not very surface active. The best representatives of these quaternary compounds bring about a surface tension lowering to 32 mN/m in a 1% aqueous solution.
The present invention is based on the surprising finding that, in contrast to general theoretical knowledge, as expressed, for example, in the theory of Neumann, selected silanes, that is, compounds with only a single silicon atom, for which, however, the ratio of hydrophilic to hydrophobic parts of the molecule is balanced, lower the surface tension of water extraordinarily effectively and, in contrast to the siloxane surfactants, are resistant to hydrolysis for days and weeks, even in acidic and alkaline media. A further and unforeseeable advantage of the inventive silanes is their complete biological degradability, which makes them particularly suitable for use as surfactants. Such a profile of properties could not be inferred from the state of the art and contradicts previously customary assumptions concerning the structural requirements, which organosilicon compounds should meet in order to exhibit surface tension-lowering properties in aqueous systems.